The present invention relates to air throttling for the charge air inlet of an internal combustion engine, where it is common practice to provide a butterfly-type vane rotatably disposed in the throttling passage of the engine charge air inlet. Butterfly-vane type air throttles provide responsive and effective valving of the engine charge air for off-idle engine operation requiring part or fully opened throttle. However, in idle operation requiring substantially closed throttle, the butterfly-vane type air throttle has the disadvantage that an extremely small movement of the throttle vane produces drastic changes in engine speed. Consequently, it has been the practice in engine design to provide only fixed speed engine idle control by adjustment of a limit stop for maintaining the butterfly-vane throttle "cracked" or opened only a very small amount from the fully closed position during engine idle conditions.
However, in order to provide adequate charge air to the engine inlet when idle speed load conditions change, the butterfly throttle vane, must be moved to vary the flow of charge air to the engine inlet. For example, when the vehicle air conditioner is cycled at idle, the substantial change in engine load requires a compensating throttle change. However, it has been found difficult to provide a vehicle operator adjustable idle speed control. Presently, it is the practice to provide a solenoid operated step for the fixed adjustment for closed throttle limit stop. The solenoid, when actuated, positions a "fast idle" cam under the closed throttle position adjustment screw ior holding the throttle open a slight additional amount to increase engine idle speed.
It has, however, been desired to provide a way or means of varying the engine change air flow in order to maintain a constant idle speed either by vehicle operator control or by automatic electric control. In particular, where engine fuel injection is controlled electrically by a microprocessor on board the vehicle, in order to control combustion to minimize engine emissions, it has been desired to provide a way or means of electrically varying the engine idle speed to provide the most efficient combustion at idle and also to accommodate changes in accessory load on the engine while running in idle mode.